The present invention generally relates to information read out apparatusses, and more particularly to a read out apparatus for reading out information from a magneto-optic disk.
Magneto-optic disks are becoming popular because of the large member capacity, high reliability and the like. Hence, the magneto-optic disk is applicable to various fields including recording and read out of image information and recording codes for use in computers.
A description will be given of the basic principle of recording and reading out information on and or from a magneto-optic disk, by referring to FIGS. 1A and 1B.
First, as shown in FIG. 1A, an external magnetic field of a magnet 11 is applied on a magneto-optic disk 10, and an erasting beam 12 is irradiated on a part where information is to be rewritten. The part which is irradiated by the erasing beam 12 is heated, and the direction of magnetization at this part is arranged in one direction.
Then, as shown in FIG. 1B, the direction of the external magnetic field of the magnet 11 is reversed. A recording light beam 13 is irradiated on the magneto-optic disk 10 depending on a recording data Dw shown in FIG. 2(A) which is to be recorded as shown in FIG. 2(B). Hence, the magneto-opticl disk 10 is selectively heated and magnetic domains D having different directions of magnetization are formed so that intervals of edge positions of the magnetic domains correspond to the recording data Dw as shown in FIG. 2(C). The magnetic domains D are formed on tracks or track turns of the magneto-optic disk 10.
There are two methods of forming the magnetic domains D. The mark position method forms the magnetic domains D in correspondence with the data "1"s of the recording data Dw. On the other hand, the edge recording method (or the mark length recording method) forms the magnetic domains D so that leading or trailing edges correspond to the data "1"s of the recording data Dw. According to the mark position method, it is difficult to improve the recording density because the recording data Dw is made to correspond as it is to the direction of magnetization. However, the edge recording method is a compression technique which makes the data "1"s of the recording data Dw correspond to the edges of the magnetic domains D, and it is possible to greatly improve the recording density.
FIG. 2(A) through (C) respectively show the recording data Dw, the light emitting pattern of the recording light beam 13 and the recorded magnetic domains D on the magneto-optic disk 10 for the case where the edge recording method is employed. Hence, the recording light beam 13 is turned ON and OFF as shown in FIG. 2(B) depending on the data "1"s of the recording data Dw shown in FIG. 2(A), so that the edges of the magnetic domains D on the magneto-optic disk 10 correspond to the data "1"s of the recording data Dw as shown in FIG. 2(C).
When reading out the recorded information from the magneto-optic disk 10, a read out light spot Pr scans the magnetic domains D as shown in FIG. 3(A). A read out signal Sr shown in FIG. 3(B) is obtained by the scan of the read out light spot Pr, and a read out data Dr shown in FIG. 3(C) can be read out by detecting the intervals of the edges of the read out signal waveform shown in FIG. 3(B).
Various systems have been proposed for reading out information from the magneto-optic disk, and examples of such systems are disclosed in Japanese Laid-Open Patent Applications No. 61-214278 and No. 63-53722.
FIG. 4 shows an example of a conventional read out system. The read out system shown in FIG. 4 includes a head 111, an amplifier 112, a signal processing circuit 113, a phase locked loop (PLL) circuit 114, a data separator 115 and a decoding circuit 116 which are connected as shown. When the read out light spot Pr scans tracks of the magneto-optic disk 10, and the read out signal Sr shown in FIG. 3(B) is output from the amplifier 112. The signal processing circuit 113 processes the read out signal Sr and outputs an edge signal Se shown in FIG. 3(D) which indicates the rising and falling edge positions of the read out signal Sr. A clock signal is formed in the PLL circuit 114 based on the edge signal Se, and the data separator 115 obtains the read out data Dr show in FIG. 3(C) based on the clock signal and the edge signal Se. Since the read out data Dr takes the form of a run length limited code suited for the recording on the magneto- optic disk 10, the read out data Dr (code) is converted into a normal digital data in the decoding circuit 116.
Generally, the edge positions of the read out signal Sr are detected by use of a threshold value L shown in FIG. 3(B). This threshold value L is a center value between maximum and minimum values of the read out signal Sr, and the intersections of the read out signal Sr and this threshold value L are detected as the edge positions of the magnetic domains D.
FIG. 5 shows a data format on the magneto-optic disk 10. In order to manage the recorded data, each track of the magneto-optic disk 10 is divided into ten-odd number of sectors. A sector mark Ms which indicates the start of the sector is recorded at the head of each sector, and an identification (ID) number Mi which specifies each sector is recorded after the sector mark Ms. The sector mark Ms and the ID number Mi are physically formed pits of .lambda./4, where .lambda. denotes the wavelength. Variable frequency oscillator (VFO) pull-in domains are recorded in a VFO pull-in area Mv and phase adjusting domains are recorded in a synchronized byte (SB) area Ms, both by magnetic means, following the ID number Mi. Further, the data is recorded in a data area Md following the SB area Ms. The VFO pull-in domains are made up of magnetic domains which have a predetermined length and are arranged at predetermined intervals.
When the operator specifies the data which is to be read out at the time of the read out, the head 111 moves to the sector which contains the specified data. Then, after confirming that the ID number of this sector matches the ID number of the target sector which contains the specified data, the read out signal Sr shown in FIG. 3(B) is obtained by reading the row of the VFO pull-in domains recorded in the VFO pull-in area Mv, and the edge positions of each of the pits are detected from the edge signal Se shown in FIG. 3(D). As described above, the VFO pull-in domains are made up of magnetic domains which have a predetermined length and are arranged at predetermined intervals. Accordingly, by supplying to the PLL circuit 114 the edge signal Se which is obtained based on the read out signal Sr of the VFO pull-in domains, it is possible to adjust the frequency of the clock signal to a predetermined frequency prior to the data read out. In addition, it is also possible to adjust the phase of the frequency-adjusted clock signal using the domains of the SB area Ms.
However, according to the edge recording method, the edges of the magnetic domains correspond to the data "1"s of the recording data. For this reason, unless the recording is made so that the length of the magnetic domains (that is, the interval of the edges) accurately matches a predetermined length (interval), there is a problem in that the decoded read out data will not match the recording data.
On the other hand, the magneto-optic disk is heated by a laser beam at the time of the recording. Hence, there is a problem in that the length of the magnetic domains (that is, the interval of the edges) becomes different at parts of the magneto-optic disk even if the recording is carried out at the same laser power, due to inconsistent heating conditions, a change in ambient temperature, non-uniform heat sensitivities at various parts of the magneto-optic disk and the like. Furthermore, there is a problem in that the length of the magnetic domains becomes different among the individual magneto-optic disks due to non-uniform heat sensitivities among the magneto-optic disks and the like.
Next, a description will be given of another example of a conventional read out system, by referring to FIG. 6. The read out system shown in FIG. 6 includes an optical head 120, an edge detection circuit 121, PLL circuits 122a and 122b, data separators 123a and 123b, buffers 124a and 124b, a synthesizing circuit 125, and a decoding circuit 126 which are connected as shown. The read out system shown in FIG. 6 is of the type described in the Japanese Laid-Open Patent Application No. 61-214278 referred above.
In FIG. 6, the read out signal which is output from the head 120 is supplied to the edge detection circuit 121 wherein the leading and trailing edges of the recorded magnetic domains are detected. A first edge detection signal of the leading edges is output from the edge detection circuit 121 and is supplied to the PLL circuit 122a which generates a clock signal synchronized thereto. Similarly, a second edge detection signal of the trailing edges is output from the edge detection circuit 121 and is supplied to the PLL circuit 122b which generates a clock signal synchronized thereto.
The data separator 123A separates the data from the first edge detection signal using the output clock signal of the PLL circuit 122a. Similarly, the data separator 123b separates the data from the second edge detection signal using the output clock signal of the PLL circuit 122b. The output data of the data separator 123A is written into the buffer 124a, while the output data of the data separator 123b is written into the buffer 124b. The data stored in the buffers 124a and 124b are read out in synchronism and synthesized in the synthesizing circuit 125. An output data of the synthesizing circuit 125 takes the form of the run length limited code (RLLC), the decoding circuit 126 decodes the output data of the synthesizing circuit 125 into a non-return-to-zero (NRZ) code signal, and this NRZ code signal is output via an output terminal 127.
As described above, the recording light beam selectively heats the magneto-optic disk 10 to form the magnetic domains by the thermal magnetic writing technique. Hence, if the ambient temperature changes, the temperature distribution on the magneto-optic disk 10 does not become uniform even if the recording light beam irradiates at the same write (laser) power. When the temperature distribution is not uniform, the size of the magnetic domains change and it no longer becomes possible to make a correct recording because the intervals of the edge positions of the magnetic domains will change.
For example, the domain length greatly changes as indicated by a solid line I in FIG. 7 depending on the write power, that is, the temperature. In addition, magneto-optic disk 10, the size of the magnetic domains still may become inconsistent due to the inconsistent sensitivity within the magneto-optic disk 10 and inconsistencies in the sensitivities among individual magneto-optic disks 10. Furthermore, the magnetic domain D which is formed has the so-called tear drop shape as shown in FIG. 2(C), and the detected position becomes different for the leading and trailing edges of the magnetic domain D due to its shape.
On the other hand, the interval between the leading edges of the adjacent magnetic domains or the interval between the trailing edges of the adjacent magnetic domains is approximately constant regardless of the write power, as indicated by a solid line II in FIG. 7. For this reason, even though the sensitivity within the magneto-optic disk 10 may be inconsistent and the detected position may become different for the leading and trailing edges of the magnetic domain D due to its shape, the read out system shown in FIG. 6 can carry out the read out so as not to be greatly affected by the change in the ambient temperature, the inconsistencies among the individual magneto-optic disks 10, the tear drop shape of the magnetic domain D and the like. This is because the read out system shown in FIG. 6 synthesizes in synchronism the data which are obtained by independently detecting the leading edges and the trailing edges of the magnetic domains.
However, there is a problem in that the read out system shown in FIG. 6 requires two circuit systems, that is, a first system including the PLL 122a, the data separator 123A and the buffer 124a for processing the leading edges of the magnetic domains, and a second system including the PLL 122b, the data separator 123b and the buffer 124b for processing the trailing edges of the magnetic domains. The scale of the circuit becomes large according to the read out system shown in FIG. 6.